Relative shortening of femur length and nuchal-fold thickening were evaluated as sonographic signs for the prenatal detection of Down syndrome. The sonograms of 28 fetuses identified by amniocentesis as having trisomy 21 were reviewed. The sonograms of 128 fetuses with a normal karyotype were evaluated as a control group. The finding of either an actual-to-expected femur-length ratio of 0.91 or less or a nuchal-fold measurement of 6 mm or more detected Down syndrome with a sensitivity of 42% and a specificity of 88%. The positive predictive value for Down syndrome detection when using femur-length shortening alone was greater than that when using the combination of femur-length shortening and nuchal-fold thickness. We conclude that femur-length shortening may be potentially useful for the detection of Down syndrome, whereas nuchal-fold thickening has a limited role.